The following two problems need to be overcome during the development of the amorphous alloy catalysts. One is how to increase the specific surface area of the amorphous alloy catalysts, so that the catalytic activity can be improved; the other is how to keep the catalyst in its amorphous state, i.e., how to enhance the thermal stability of the amorphous alloy catalyst. There have been numerous attempts to provide solutions to this problem.
In CN 1,073,726A, an alloy containing Al, rare earth elements (RE), P and Ni or Co or Fe was prepared by rapid quenching techniques. By alkaline leaching of Al from the alloy, using NaOH, a Ni/Co/Fe--RE--P amorphous alloy catalyst with high specific surface area of 50-130 m.sup.2 /g was obtained. Its hydrogenation activity was greater than that of Raney Ni catalyst, which is widely used in industry. Such a catalyst exhibited the highest reactivity among all the prior art amorphous alloy catalysts reported so far.
An ultra-fine Ni--B amorphous alloy catalyst was reported by Deng et al., J. Catal. 150,434-438 (1994). This catalyst was prepared by adding 2.5 M aqueous KBH.sub.4 solution dropwise, with stirring at 25.degree. C., to an 0.1M nickel acetate (Ni(CH.sub.3 COO).sub.2) alcoholic solution. The resultant Ni--B deposit was first washed with 6 ml of 8M ammonium hydroxide (NH.sub.4 OH), and then with a large amount of distilled water. However, such ultra-fine Ni--B amorphous alloy particles exhibited poor thermal stability, although their specific surface area was as high as 29.7 m.sup.2 /g. Deng et al., also reported that ultra-fine Ni--P amorphous alloy catalyst can be prepared by heating an aqueous solution containing nickel acetate, sodium acetate (CH.sub.3 COONa), and sodium hypophosphite (NaH.sub.2 PO.sub.2) at 90.degree. C., and adjusting its pH to 11 with an aqueous solution of NaOH while stirring, and then sequentially washing the resulting deposit with ammonium hydroxide, and distilled water. The specific surface area of this Ni--P amorphous alloy catalyst is 2.78 m.sup.2 /g only, while its maximal crystallization temperature increases to 394.4.degree. C.
It was reported by Dichang et al., in Acta Physico-Chimica Sinica, 9(3), 325-330 (1993), that by introducing rare earth elements, such as Y, Ce and Sm, the resulting unsupported Ni--RE--P amorphous alloy catalysts exhibited a much better thermal stability than the corresponding unsupported Ni--P amorphous alloy catalyst. Similar results were also obtained by introducing La instead of the above rare earth elements, as reported in J. Chem. Soc. Faraday Trans. I, 82, 702 (1986).
The effects of the metal additives, such as Pd, Co, Cu, and Fe, on the hydrogenation properties of the Ni--B amorphous alloy catalysts were studied by Bingshi et al., as reported in Petrochemical Technology 23(2), 791-794 (1994). The results demonstrated that the addition of Pd could enhance the activity of the Ni--B amorphous alloy catalyst for cyclopentadiene hydrogenation, while other metal additives including Co, Cu, or Fe reduced the hydrogenation activity.
Chen et al., reported in Acta Chimica Sinica 52, 877-882 (1994) that an unsupported Ni--W--P amorphous alloy catalyst can be prepared by heating an aqueous solution containing nickel acetate Ni(CH.sub.3 COO).sub.2, sodium acetate (CH.sub.3 COONa), sodium hypophosphite (NaH.sub.2 PO.sub.2), and sodium tungstate (Na.sub.2 WO.sub.4) to 90.degree. C., and adjusting its pH to 10-11 with aqueous NaOH with stirring, and sequentially washing the resulting deposit with ammonium hydroxide, distilled water and anhydrous ethanol. The Ni--W--P catalyst has a higher cyclopentadiene hydrogenation rate compared to a Ni--P catalyst, and has a lower cyclopentene hydrogenation rate than that of a Ni--P catalyst.
An Ni--P amorphous alloy catalyst supported on silica was reported by Jingfa & Xiping in Appl. Catal. 37, 339-343 (1988). The catalyst was prepared by chemical plating, i.e., by mixing a solution containing sodium citrate (Na.sub.3 C.sub.6 H.sub.5 O.sub.7), nickel sulfate (NiSO.sub.4), sodium hypophosphite (NaH.sub.2 PO.sub.2), and sodium acetate (CH.sub.3 COONa) with a silica gel carrier, heating to 363.degree. K with stirring, and keeping the pH at 5.0 for about 2 hours. The product was then washed with distilled water, and dried overnight. Such a supported Ni--P amorphous alloy catalyst exhibited not only a high specific surface area up to 85 m.sup.2 /g, but also a superior thermal stability (maximal crystallization temperature is 352.degree. C.). However, such a supported Ni--P amorphous alloy catalyst exhibited the following disadvantages:
(1) This catalyst is prepared by mixing a SiO.sub.2 carrier with a solution containing Na.sub.3 C.sub.6 H.sub.5 O.sub.7, CH.sub.3 COONa, NiSO.sub.4, and NaH.sub.2 PO.sub.2 and heating with stirring. Na.sub.3 C.sub.6 H.sub.5 O.sub.7 and CH.sub.3 COONa, as a complexing agent of Ni.sup.2+, function by controlling the concentration of Ni.sup.2+ in solution. The pH value adjusts the reduction rate of Ni.sup.2+, and the formation rate of the Ni--P amorphous alloy. That is to say, the formation rate of Ni--P amorphous alloy is slow due to the pH value of 5.0. The existence of Na.sub.3 C.sub.6 H.sub.5 O.sub.7 and CH.sub.3 COONa, is advantageous for depositing Ni--P amorphous alloy on the SiO, carrier. However, because the reduction of Ni.sup.2+ is conducted in solution, only a small proportion of the Ni--P amorphous alloy is actually deposited onto, and supported by, the carrier itself. Most of the Ni--P amorphous alloy is left on the wall or the bottom of the container, so that the yield of the supported Ni--P amorphous alloy on the SiO.sub.2 carriers is very low. Furthermore, Ni(Fe/Co)--B and Ni(Fe/Co)--P can itself function as a catalyst in Ni.sup.2+ reduction (see Shen et al. J. Phys. Chem 97, 8504-8511, (1993)). Therefore, since the amount of Ni--P amorphous alloy not supported on the SiO.sub.2 carrier exceeds that supported by SiO.sub.2 carrier, the reduction and deposition rate of Ni--P amorphous alloy on the wall or at bottom of the container is in turn further enhanced, so that the proportion of Ni--P amorphous alloy supported on the SiO, carriers decreases still further.
(2) Na.sub.3 C.sub.6 H.sub.5 O.sub.7 and CH.sub.3 COONa, as a complexing agent of Ni.sup.2+ can control the reaction rate of Ni.sup.2+, and is advantageous for supporting Ni--P amorphous alloy on the carrier. However, due to shielding of the complexing agent, the degree of reduction of Ni.sup.2+ is lowered, so that a proportion of the Ni.sup.2+ cannot be reduced by H.sub.2 PO.sub.2.sup.-. Thus, the cost for producing such a catalyst increases.
(3) The activity of the catalyst does not meet expected levels. Since the Ni(Fe/Co)--B and Ni(Fe/Co)--P can function as catalysts in Ni.sup.2+ reduction, if the above-mentioned amorphous Ni--P alloy is pre-supported homogeneously throughout the carrier, Ni.sup.2+ will be catalytically reduced inside the carrier by H.sub.2 PO.sub.2.sup.-. Thus, a majority of the Ni--P amorphous alloy will be uniformly absorbed onto the carrier with the assistance of induction and catalysis by Ni(Fe/Co)--B and Ni(Fe/Co)--P. However, when a catalyst was prepared according to the process disclosed in Jingfa & Xiping, Appl. Catal. 37 339-343 (1988), the reduction is substantially conducted in solution. The Ni--P amorphous alloy consequently formed will be supported predominantly on the surface of the carrier. It is, therefore, difficult to obtain a product wherein the Ni--P amorphous alloy is uniformly deposited and supported throughout the carrier. In addition, there are no reports in the prior art of Ni--P amorphous alloy catalysts uniformly supported on a carrier.
According to Shen et al., J. Phys. Chem. 97, 8504-8511 (1993), the reaction between the metal ion M.sup.2+ and the reducing agent BH.sub.4.sup.- in an aqueous solution follows the three independent reactions given below: EQU BH.sub.4.sup.- +2H.sub.2 O=BO.sub.2.sup.- +4H.sub.2 .uparw. EQU BH.sub.4.sup.- +2M.sup.2 +2H.sub.2 O=2M .dwnarw.+BO.sub.2.sup.- +4H.sup.+ +2H.sub.2 .uparw. EQU BH.sub.4.sup.- +H.sub.2 O=B+OH.sup.- +2.5H.sub.2 .uparw.
Since all three reactions proceed very rapidly, it could not be guaranteed that the resulting Ni--B amorphous alloy could be deposited efficiently on the carrier when the chemical plating is employed, when the carrier and the plating solution were mechanically mixed. Also, the homogeneous distribution of Ni--B amorphous alloy on the carrier could not be achieved. Therefore, there is a need to deposit and support Ni--B amorphous alloy uniformly on and throughout a carrier.